Adhesive layer forming liquid and adhesive layer forming process

ABSTRACT

The object of the invention is to provide an adhesive layer forming liquid capable of maintaining the performance of forming an adhesive layer easily and keeping adhesive property to a resin certainly, and an adhesive layer forming process using the liquid. The adhesive layer forming liquid of the invention is an adhesive layer forming liquid for forming an adhesive layer for bonding a copper and a resin to each other, which is an aqueous solution comprising an acid, a stannous salt, a stannic salt, a complexing agent, and a stabilizer, and which is prepared to set the value of B/A to 0.010 or more and 1.000 or less at the time of the preparation, wherein A represents the concentration (unit: % by mass) of the stannous salt as the concentration of bivalent tin ions, and B represents the concentration (unit: % by mass) of the stannic salt as the concentration of tetravalent tin ions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an adhesive layer forming liquid for forming an adhesive layer for bonding a copper and a resin to each other, and an adhesive layer forming process using this liquid.

2. Description of the Related Art

Ordinary multilayer interconnection boards are each produced by laminating and pressing an internal substrate, which has, on its surface, an electroconductive layer made of copper, and one or more different internal substrates and/or one or more copper foil layers to sandwich one or more prepregs therebetween. The electroconductive layers are connected electrically to each other through open holes called through holes, the hole-walls of which are plated with copper. On the surface of the electroconductive layer of each of the internal substrates, a tin plating layer may be made from a tin plating solution described in Japanese Patent Application Publication (JP-B) No. 6-66553 and Japanese Patent Application Translated-Version National Publication No. 2004-536220 and others in order to improve the adhesive property of the layer surface onto the prepreg adjacent thereto.

However, about the tin plating solution described in Japanese Patent Application Publication (JP-B) No. 6-66553 and Japanese Patent Application Translated-Version National Publication No. 2004-536220, a stannous salt is used as a tin source; thus, a bivalent tin ion (Sn²⁺) is oxidized to a tetravalent tin ion (Sn⁴⁺) by air oxidation or the like when the solution is used. As a result, there is caused a problem that the plating adhesive property is lowered so that the closely adhesive property between tin and a resin is declined.

Against the problem, Japanese Patent Application Laid-Open (JP-A) No. 5-222540 and JP-A No. 5-263258 suggest a method of regenerating a bivalent tin ion from a tetravalent tin ion by use of metallic tin.

-   Patent Document 1: Japanese Patent Application Publication (JP-B)     No. 6-66553 -   Patent Document 2: Japanese Patent Application Translated-Version     National Publication No. 2004-536220 -   Patent Document 3: Japanese Patent Application Laid-Open (JP-A) No.     5-222540 -   Patent Document 4: Japanese Patent Application Laid-Open (JP-A) No.     5-263258

SUMMARY OF THE INVENTION

However, according to the above method, the adjustment of components in the tin plating solution is difficult. Thus, the method is poor in practicability.

In light of the actual situation, the present invention has been made, and an object thereof is to provide an adhesive layer forming liquid capable of maintaining the performance of forming an adhesive layer easily and keeping adhesive property to a resin certainly, and an adhesive layer forming process using the liquid.

The adhesive layer forming liquid of the present invention is an adhesive layer forming liquid for forming an adhesive layer for bonding a copper and a resin to each other, which is an aqueous solution comprising an acid, a stannous salt, a stannic salt, a complexing agent, and a stabilizer, and which is prepared to set the value of B/A to 0.010 or more and 1.000 or less at the time of the preparation, wherein A represents the concentration (unit: % by mass) of the stannous salt as the concentration of bivalent tin ions, and B represents the concentration (unit: % by mass) of the stannic salt as the concentration of tetravalent tin ions.

The adhesive layer forming process of the present invention is an adhesive layer forming process for forming an adhesive layer for bonding a copper and a resin to each other, comprising the step of processing a surface of the copper with the adhesive layer forming liquid as recited in the above adhesive layer forming liquid.

In the present invention, “copper” may be copper or any copper alloy. In the present specification, the word “copper” denotes copper or any copper alloy.

According to the adhesive layer forming liquid of the present invention, and the adhesive layer forming process using this liquid, it is possible to maintain the performance of forming an adhesive layer easily and keep adhesive property to a resin certainly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to an adhesive layer forming liquid for forming an adhesive layer made mainly of copper-tin alloy on a surface of copper in order to bond the copper and a resin to each other, and an adhesive layer forming process using the liquid. The copper surface is, for example, a surface of a copper foil layer (such as an electrolytic copper foil layer or rolled copper foil layer) used for a semiconductor wafer, an electronic substrate, an electronic component such as a lead frame, an ornament, or a building member; a surface of a copper plating film (such as an electroless plating copper film or electroplating copper film) ; or a surface of a copper member which may be in various forms such as linear, rodlike, tubular, and tabular forms. Hereinafter, components contained in the adhesive layer forming liquid of the present invention will be described in detail.

(Acid)

The acid contained in the adhesive layer forming liquid of the present invention functions as a pH adjustor and a tin ion stabilizer. Examples of the acid include inorganic acids, such as hydrochloric acid, sulfuric acid, nitric acid, fluoroboric acid, and phosphoric acid; and water-soluble organic acids, such as carboxylic acids such as formic acid, acetic acid, propionic acid and butyric acid, alkanesulfonic acids such as methanesulfonic acid and ethanesulfonic acid, and aromatic sulfonic acids such as benzenesulfonic acid, phenolsulfonic acid and cresolsulfonic acid. Of the acids, sulfuric acid and hydrochloric acid are preferred from the viewpoint of the rate of forming the adhesive layer, the solubility of the stannic salt therein, and the like. The concentration of the acid is preferably from 1 to 50% by mass, more preferably from 3 to 30% by mass, even more preferably from 5 to 20% by mass. When the concentration is in the range, an adhesive layer excellent in closely adhesive property can easily be formed.

(Stannous Salt)

In the present invention, a stannous salt is used as a source for tin. Examples of the stannous salt include stannous sulfate, stannous borofluoride, stannous fluoride, stannous nitrate, stannous oxide, stannous chloride, stannous formate, and stannous acetate. The concentration of the stannous salt is preferably from 0.05 to 10.00% by mass, more preferably from 0.10 to 5.00% by mass, and even more preferably from 0.50 to 3.00% by mass as the concentration of bivalent tin ions. When the concentration is in this range, an adhesive layer excellent in adhesive property can easily be formed.

(Stannic Salt)

At the time of the preparation of the adhesive layer forming liquid of the present invention, to the liquid is added a stannic salt besides the stannous salt. This makes it possible to restrain the bivalent tin ions in the adhesive layer forming liquid from being oxidized into tetravalent tin ions by oxidation with air or the like even when the liquid is continuously used. The stannic salt may be selected from stannic salts soluble in acidic solution, and used without particular restriction. From the viewpoint of solubility, the stannic salt is preferably a salt of tin with the above-mentioned acid. Examples thereof include stannic sulfate, stannic fluoride, stannic nitrate, stannic chloride, stannic formate, and stannic acetate. The concentration of the stannic salt is preferably from 0.01 to 5.00% by mass, and more preferably from 0.01 to 1.50% by mass as the concentration of tetravalent tin ions. When the concentration is in this range, an adhesive layer excellent in adhesive property can easily be formed.

The adhesive layer forming liquid of the present invention is prepared to set the value of B/A into the range of 0.010 or more and 1.000 or less at the time of the preparation, wherein A represents the concentration (unit: % by mass) of the stannous salt as the concentration of bivalent tin ions, and B represents the concentration (unit: % by mass) of the stannic salt as the concentration of tetravalent tin ions. The wording “at the time of the preparation” denotes the time when the individual components constituting the adhesive layer forming liquid are blended with each other, specifically, the time when the stannous salt and the stannic salt are blended with each other. By setting the value of B/A to 0.010 or more at the time of the preparation of the adhesive layer forming liquid, the performance of forming an adhesive layer can easily be maintained even when the liquid is continuously used. By setting the value of B/A to 1.000 or less at the time of the preparation of the adhesive layer forming liquid, the liquid can certainly keep adhesive property to a resin. In order to exhibit the advantageous effects more effectively, it is preferred to set the value of B/A to 0.050 or more and 0.500 or less at the time of the preparation of the adhesive layer forming liquid.

(Complexing Agent)

The complexing agent contained in the adhesive layer forming liquid of the present invention is an agent which is coordinated to the copper layer as an underlying layer to form a chelate, thereby making it easy to form the adhesive layer on the surface of the copper layer. For example, thioureas such as thiourea, 1,3-dimethylthiorea and 1,3-diethyl-2-thiourea; and thiourea derivatives such as thioglycolic acid may be used. The concentration of the complexing agent ranges preferably from 1 to 30% by mass, more preferably from 1 to 20% by mass. When the concentration is in this range, an adhesive layer excellent in closely adhesive property can easily be formed without lowering the rate of forming the adhesive layer. Moreover, when the concentration is in this range, an adhesive layer good in smoothness can be formed.

(Stabilizer)

The stabilizer contained in the adhesive layer forming liquid of the present invention is an additive for maintaining the concentrations of the individual components necessary for the reaction in the vicinity of the surface of the copper layer. Examples thereof include such as glycols such as ethylene glycol, diethylene glycol, propylene glycol and tripropylene glycol; and glycol esters such as cellosolve, carbitol and butyl carbitol. The concentration of the stabilizer ranges preferably from 1 to 80% by mass, more preferably from 5 to 80% by mass, and even more preferably from 10 to 80% by mass. When the concentration is in the range, the concentrations of the individual components necessary for the reaction can easily be maintained in the vicinity of the surface of the copper layer.

To the adhesive layer forming liquid of the present invention may be appropriately added a reducing agent such as hypophosphorous acid, a brightener, a pH adjustor, a preservative, and/or other(s) as the need arises. The content of these additives is, for example, from about 0.1 to 20% by mass.

The adhesive layer forming liquid of the present invention can easily be prepared by dissolving the above-mentioned individual components into water. The water is preferably water from which ionic materials and impurities are removed, and preferred examples thereof include ion-exchange water, pure water, and ultra pure water.

In the case of using the adhesive layer forming liquid to form the adhesive layer, the formation can be attained under conditions described below.

First, the surface of the copper layer is washed with an acid or the like. Next, the copper layer is immersed into the adhesive layer forming liquid, and then subjected to swinging immersion treatment for 5 seconds to 5minutes. At this time, it is advisable that the temperature of the adhesive layer forming liquid is from 20 to 70° C. (preferably 20 to 40° C.) Thereafter, the treated surface is rinsed with water and dried, thereby forming the adhesive layer.

Furthermore, the surface of the adhesive layer may be treated with a tin stripping solution. When the tin stripping solution is brought into contact with the adhesive layer surface, the layer can be rendered a smoother and thinner adhesive layer.

The above tin stripping solution may be any solution that is capable of etching tin. For example, an acidic solution or the like may be used, examples thereof including such as an aqueous nitric acid solution, hydrochloric acid, an aqueous sulfuric acid solution, and mixed solutions thereof. The concentration of the acid in the acidic solution ranges preferably from 0.1 to 10% by mass, more preferably from 0.3 to 5% by mass. When the concentration is in this range, the thickness of the adhesive layer can easily be controlled into an appropriate range. An aqueous nitric acid solution is particularly preferred since the solution gives a large stripping rate. To the tin stripping solution may be added any other additive, such as a surfactant or a pH adjustor.

In the surface-stripping step, the period when the adhesive layer surface and the tin stripping solution (preferably, an aqueous nitric acid solution) contact each other is preferably from 5 to 120 seconds, more preferably from 10 to 30 seconds. When the period is in the range, the thickness of the adhesive layer can easily be controlled into an appropriate range. The method for bringing the tin stripping solution into contact with the surface may be immersion, or liquid-contacting treatment using a spray or the like. The temperature of the tin stripping solution in the method is from about 25 to 35° C.

The thickness of the adhesive layer is appropriately 0.02 μm or less, and is preferably from 0.001 to 0.02 μm, more preferably from 0.003 to 0.02 μm. When the thickness is set to 0.001 μm or more, the adhesive property to a resin layer can be certainly kept with ease. On the other hand, when the thickness of the adhesive layer is set to 0.02 μm or less, the adhesive layer can easily be removed in a case where the adhesive layer is required to be removed in a subsequent step.

The constituting resin of a resin layer to be bonded to the adhesive layer is not particularly limited. Examples of the resin include thermoplastic resins such as acrylonitrile/styrene copolymer resins (AS resins), acrylonitrile/butadiene/styrene terpolymer resins (ABS resins), fluorine-contained resins, polyamide, polyethylene, polyethylene terephthalate, polyvinylidene chloride, polyvinyl chloride, polycarbonate, polystyrene, polysulfone, polypropylene, and liquid crystal polymers; thermosetting resins such as epoxy resins, phenol resins, polyimide, polyurethane, bismaleimide/triazine resins, modified polyphenylene ether, and cyanate esters; and ultraviolet curable resins such as ultraviolet curable epoxy resins and ultraviolet curable acrylic resins. These resins may be modified with a functional group, or may be reinforced with glass fiber, aramide fiber, some other fiber, or the like.

The adhesive layer obtained from the adhesive layer forming liquid of the present invention can ensure the adhesive property of the layer to an insulating resin, an etching resist, a solder resist, an electroconductive resin, an electroconductive paste, an electroconductive adhesive, a dielectric resin, a hole-filling resin, a flexible coverlay film, or the like. Thus, according to the present invention, adhesive property between a copper layer and a resin layer can be ensured. Accordingly, for example, a wiring board high in reliability can be supplied.

Examples

Examples of the invention will be described together with comparative examples hereinafter. The invention should not be construed to be limited to the examples.

(Treatment with Fresh Liquids)

Adhesive layer forming liquids (temperature: 30° C.) having compositions in Table 1-1 described below, respectively, were each prepared by 1 liter. About each of the adhesive layer forming liquids, the balance other than the components shown in Table 1-1 was made of ion-exchange water. Electroplating copper foil pieces (trade name: 3EC-III, manufactured by Mitsui Mining Co., Ltd.; thickness: 35 μm) each cut into a size of 100 mm×100 mm were prepared as test pieces. Any one of the test pieces was put into each of the liquids (fresh liquids), and then the piece was subjected to swinging immersion treatment for 30 seconds. Thereafter, the treated test piece was washed with water, and immediately the piece was subjected to swinging immersion treatment with a 0.7% by mass of an aqueous nitric acid solution (temperature: 30° C.) for 20 seconds. Thereafter, the piece was washed with water, and dried.

(Treatment with Used Liquids)

Separately from the above, adhesive layer forming liquids (temperature: 30° C.) having the compositions in Table 1-1 described below, respectively, were each prepared by 1 liter. While each of the adhesive layer forming liquids was stirred, 500 pieces of the same test pieces as described above were continuously treated therewith under the same conditions as described above over 24 hours. Next, any one of the same test pieces as described above was put into each of the liquids (used liquids) used for the treatment, and then treated under the same conditions as described above. Thereafter, the treated test pieces were each washed with water, and immediately the piece was subjected to swinging immersion treatment with a 0.7% by mass of an aqueous nitric acid solution (temperature: 30° C.) for 20 seconds. Thereafter, the piece was washed with water, and dried.

(Adhesive Property)

Each of the treated test pieces was sampled, and then a solution of a photosensitive liquid solder resist (trade name: SR-7200, manufactured by Hitachi Chemical Co., Ltd.) was applied into a thickness of about 20 μm onto the sample test piece so as to interpose the adhesive layer therebetween, and then the resist was cured. Thereafter, according to JIS C 6471, the peel strength (N/mm) thereof was measured. The results are shown in Table 1-2.

(Tin Plating Adhesiveness)

Prepared were 1 liter of a fresh liquid (temperature: 30° C.) of an adhesive layer forming liquid having each composition shown in Table 1-1 described below; and 1 liter of a used liquid (temperature: 30° C.) of a liquid having the same composition, with which 500 electroplating copper foil pieces were continuously treated under the same conditions as described above over 24 hours. As test pieces, prepared were electroplating copper foil pieces (trade name: 3EC-III, manufactured by Mitsui Mining Co., Ltd.; thickness: 35 μm) each cut into a size of 100 mm×100 mm. The test pieces were put one by one into each of the liquids (the fresh liquid and the used liquid). The external appearances thereof were observed to evaluate the tin plating adhesiveness (performance of forming an adhesive layer) of each of the adhesive layer forming liquids. A case where a plating layer was formed in less than 3 seconds is represented by ◯; a case where a plating layer was formed in a period of 3 to 10 seconds, Δ; and a case where a plating layer was not formed in 10 seconds, ×.

TABLE 1-1 Liquid composition (concentration (unit: % by mass)) Stannous Stannic Complexing Acid salt salt agent Stabilizer Other component Example 1 Hydrochloric Stannous Stannic Thiourea Diethylene — acid sulfate chloride (4.4) glycol (13.9) *1 (1.40) *2 (0.02) *3 (40.0) Example 2 Hydrochloric Stannous Stannic Thiourea Diethylene — acid sulfate chloride (4.4) glycol (13.9) *1 (1.40) *2 (0.05) *3 (40.0) Example 3 Hydrochloric Stannous Stannic Thiourea Diethylene — acid sulfate chloride (4.4) glycol (13.9) *1 (1.40) *2 (0.08) *3 (40.0) Example 4 Hydrochloric Stannous Stannic Thiourea Diethylene — acid sulfate chloride (4.4) glycol (13.9) *1 (1.40) *2 (0.20) *3 (40.0) Example 5 Hydrochloric Stannous Stannic Thiourea Diethylene — acid sulfate chloride (4.4) glycol (13.9) *1 (1.40) *2 (0.70) *3 (40.0) Example 6 Hydrochloric Stannous Stannic Thiourea Diethylene — acid sulfate chloride (4.4) glycol (13.9) *1 (1.40) *2 (1.20) *3 (40.0) Example 7 Hydrochloric Stannous Stannic Thiourea Diethylene — acid sulfate chloride (4.4) glycol (13.9) *1 (1.40) *2 (1.40) *3 (40.0) Example 8 Sulfuric Stannous Stannic Thiourea Diethylene — acid sulfate chloride (4.4) glycol (13.9) (1.40) *2 (0.20) *3 (40.0) Example 9 Hydrochloric Stannous Stannic 1,3-Dimethyl Diethylene — acid sulfate chloride thiourea glycol (13.9) *1 (1.40) *2 (0.20) *3 (4.4) (40.0) Example 10 Hydrochloric Stannous Stannic Thiourea Tripropylene — acid sulfate chloride (4.4) glycol (13.9) *1 (1.40) *2 (0.20) *3 (40.0) Example 11 Hydrochloric Stannous Stannic Thiourea Diethylene Hypophosphorous acid sulfate chloride (4.4) glycol acid (13.9) *1 (1.40) *2 (0.20) *3 (40.0) (10.0) Comparative Hydrochloric Stannous — Thiourea Diethylene — Example 1 acid sulfate (4.4) glycol (13.9) *1 (1.00) *2 (40.0) Comparative Hydrochloric Stannous Stannic Thiourea Diethylene — Example 2 acid sulfate chloride (4.4) glycol (13.9) *1 (1.40) *2 (0.01) *3 (40.0) Comparative Hydrochloric Stannous Stannic Thiourea Diethylene — Example 3 acid sulfate chloride (4.4) glycol (13.9) *1 (1.40) *2 (2.00) *3 (40.0) Comparative Hydrochloric Stannous — Thiourea Diethylene — Example 4 acid sulfate (4.4) glycol (13.9) *1 (1.60) *2 (40.0) *1 Hydrogen chloride concentration *2 Bivalent tin ion concentration *3 Tetravalent tin ion concentration

TABLE 1-2 Peel strength Tin plating Concentration (N/mm) adhesiveness ratio Fresh Used Fresh Used (Sn⁴⁺/Sn²⁺) liquid liquid liquid liquid Example 1 0.014 1.10 0.82 ◯ Δ Example 2 0.036 1.11 0.93 ◯ Δ Example 3 0.057 1.10 1.02 ◯ ◯ Example 4 0.143 1.12 1.05 ◯ ◯ Example 5 0.500 1.07 1.02 ◯ ◯ Example 6 0.857 0.98 0.95 ◯ ◯ Example 7 1.000 0.97 0.92 Δ Δ Example 8 0.143 1.10 1.04 ◯ ◯ Example 9 0.143 1.07 1.01 ◯ ◯ Example 10 0.143 1.11 1.05 ◯ ◯ Example 11 0.143 1.10 1.07 ◯ ◯ Comparative — 1.02 0.40 ◯ X Example 1 Comparative 0.007 1.08 0.75 ◯ Δ Example 2 Comparative 1.429 0.77 0.73 X X Example 3 Comparative — 1.04 0.04 ◯ X Example 4

As shown in Table 1-2, about Examples 1 to 11 of the present invention, better results were obtained in each of peel strength (adhesive property) and tin plating adhesiveness than about Comparative Examples 1 to 4.

(Effect of Restraining the Oxidation of Sn²⁺ with Air by Addition of Sn⁴⁺)

In order to verify that the oxidation of Sn²⁺ with air is restrained by adding Sn⁴⁺ beforehand to a predetermined adhesive layer forming liquid, the following experiments were made:

Prepared was 1 liter of an adhesive layer forming liquid having the same composition as in Comparative Example 4 in Table 1-1. This liquid was stirred at 30° C. for 2 hours in the state that no test piece was treated with the liquid. The stirred liquid was used as an adhesive layer forming liquid (fresh liquid) of Comparative Example 5. As a test piece, prepared was an electroplating copper foil piece (trade name: 3EC-III, manufactured by Mitsui Mining Co., Ltd. ; thickness: 35 μm) cut into a size of 100 mm×100 mm. The test piece was put into the fresh liquid of Comparative Example 5, and then subjected to swing immersion treatment for 30 seconds. Thereafter, the treated test piece was washed with water, and immediately subjected to swing immersion treatment with a 0.7% by mass solution of nitric acid in water (solution temperature: 30° C.) for 20 seconds. Thereafter, the test piece was washed with water, and dried.

Separately from the above, 1 liter of the fresh liquid of Comparative Example 5 was prepared. While this liquid was stirred, 500 of the same test pieces as described above were continuously treated under the same conditions as described above over 24 hours. Next, the same test piece as described above was put into the liquid after the treatment (used liquid), and then treated under the same conditions as described above. Thereafter, the treated test piece was washed with water, and immediately subjected to swing immersion treatment with a 0.7% by mass solution of nitric acid in water (solution temperature: 30° C.) for 20 seconds. Thereafter, the test piece was washed with water, and dried.

About each of the treated test pieces, the adhesive property thereof was evaluated in the same way as described above. About each of the fresh liquid and the used liquid of Comparative Example 5, the tin plating adhesiveness was evaluated in the same way as described above. Results of these evaluations are shown in Table 2-2 together with the results of Example 4.

TABLE 2-1 Concentrations (% by mass) of individual components Hydrochloric Diethylene acid *1 Sn²⁺ Sn⁴⁺ Thiourea glycol Example 4 Fresh 13.9 1.40 0.20 4.4 40.0 liquid Used 13.9 1.20 0.40 4.4 40.0 liquid Comparative Fresh 13.9 1.40 0.20 4.4 40.0 Example 5 liquid Used 13.9 0.50 1.10 4.4 40.0 liquid *1 Hydrogen chloride concentration

TABLE 2-2 Concentration Peel ratio strength Tin plating (Sn⁴⁺/Sn²⁺) (N/mm) adhesiveness Example 4 Fresh 0.143 1.12 ◯ liquid Used 0.333 1.05 ◯ liquid Comparative Fresh 0.143 1.04 ◯ Example 5 liquid Used 2.200 0.04 X liquid

As shown in Table 2-1 and 2-2, it is understood that in Example 4, wherein Sn⁴⁺ was beforehand add, a decrease in the concentration of Sn²⁺ by oxidation with air was restrained and a change in the value of the concentration of Sn⁴⁺to that of

Sn²⁺ (i.e., the value of B/A) was also restrained. Thus, about each of the fresh liquid and the used liquid, the adhesive property and the tin plating adhesiveness were good. On the other hand, in Comparative Example 5, which contained Sn⁴⁺ generated by the oxidation of Sn²⁺ with air, a remarkable decrease in the concentration of Sn²⁺ (remarkable increase in Sn⁴⁺) by oxidation with air was observed. As a result, about the used liquid of Comparative Example 5, the adhesive property and the tin plating adhesiveness were each deteriorated. From this result, it is understood that in a case where Sn⁴⁺ is beforehand added to a predetermined adhesive layer forming liquid, the effect of preventing a deterioration in the adhesive layer forming liquid is obtained while in a case where Sn⁴⁺ is generated by the oxidation of Sn²⁺ with air, the effect of preventing a deterioration in the adhesive layer forming liquid is not obtained. 

1. An adhesive layer forming liquid for forming an adhesive layer for bonding a copper and a resin to each other, which is an aqueous solution comprising an acid, a stannous salt, a stannic salt, a complexing agent, and a stabilizer, and which is prepared to set the value of B/A to 0.010 or more and 1.000 or less at the time of the preparation, wherein A represents the concentration (unit: % by mass) of the stannous salt as the concentration of bivalent tin ions, and B represents the concentration (unit: % by mass) of the stannic salt as the concentration of tetravalent tin ions.
 2. The adhesive layer forming liquid according to claim 1, wherein A is from 0.05 to 10.00, and B is from 0.01 to 5.00.
 3. The adhesive layer forming liquid according to claim 1, wherein the complexing agent is at least one selected from thioureas and thiourea derivatives.
 4. The adhesive layer forming liquid according to claim 1, wherein the stabilizer is at least one selected from glycols and glycol esters.
 5. An adhesive layer forming process for forming an adhesive layer for bonding a copper and a resin to each other, comprising the step of processing a surface of the copper with the adhesive layer forming liquid as recited in claim
 1. 6. The adhesive layer forming process according to claim 5, wherein the step of processing the copper surface is the step of immersing the copper surface into the adhesive layer forming liquid.
 7. The adhesive layer forming process according to claim 5, further comprising the step of processing, with a tin stripping solution, the surface of the adhesive layer obtained after the step of processing the copper surface.
 8. The adhesive layer forming process according to claim 7, wherein the tin stripping solution is a solution of nitric acid in water. 